Gaseous discharge waveguide lasers such as carbon dioxide (CO.sub.2) lasers are well known in the art. Such lasers are typically constructed by providing a sealed tube having parallel mirrors at each end of the tube, and an anode and a cathode. For a laser discharge tube length of 8 cm (for example), discharge of the CO.sub.2 gas mixture in the tube may be initiated by applying a 15 kilovolt (kV) potential between the anode and the cathode. After initiation of the gas discharge, a 7 kV sustaining potential is maintained between the anode and the cathode while the 15 kV starting potential is removed. It should be recognized by those skilled in the art that the sustaining and starting potentials are a function of the discharge length and therefore, for different lengths, different potentials must be applied. Application of voltages substantially in excess of 15 kV can cause insulation breakdown of external cables or of the anode and cathode. Therefore, for gas discharge lasers of substantially longer length, the laser tube is divided into a plurality of sections, each section having its own anode and cathode, so that the ignition voltage does not have to be increased above 15 kV with an increase in the length of the gas discharge tube. This arrangement is not ideal because the space between adjoining anodes or cathodes of adjacent sections is occupied by cold gas which does not contribute to the power output of the laser and, furthermore, absorbs energy from the optical beam of the laser. Also, it is difficult to maintain a sealed vacuum within the laser tube because the increased number of anodes and cathodes increases the number of required seals.
The foregoing disadvantages are now being overcome by permitting the two adjoining sections to share a common anode which is grounded. If the laser is divided into two sections, a cathode is disposed at either end of the laser tube while a grounded common anode is located in the middle of the laser tube, thus eliminating the wasted space between individual anodes of adjoining sections. A high voltage ignition source and a sustaining voltage source are applied to each cathode at either end of the laser tube. Because the parallel aligned laser mirrors are also located at either end of the laser tube, a high voltage is induced on each of the mirrors due to the close proximity of the high voltage cathodes, presenting a severe inconvenience whenever the mirrors must be adjusted during laser operation. However, the high voltage induced on the mirrors may be eliminated as a source of inconvenience by grounding the two cathodes and applying high voltage instead to the common anode in the middle of the laser tube. Although in this latter arrangement the laser mirrors may be conveniently adjusted because they reside at ground potential, the two laser sections sharing the common anode will no longer reliably start upon application of the 15 kV ignition voltage to the common anode. This is because the two discharge sections of the laser do not start simultaneously. The high voltage source is typically a capacitive discharge device of the type well known in the art which is pulled to ground potential by the flow of current in the tube upon ignition of the gas in either one of the two sections. Therefore, when one section has ignited upon application of the ignition voltage, the voltage appearing between the electrodes is no longer sufficient to start the other section, so that only one section of the laser will be active, the other section remaining dormant. This problem may be overcome by attempting to ignite the laser in an excessive number of attempts until both sections start simultaneously by chance. However, the unreliability of this procedure is highly inconvenient